Easy-open can ends are widely used for canning many beverages and food products. An easy-open can end typically has a tear panel which can be partially or completely separated from the remainder of the can end to create an opening in the end, and an attached operating tab which may be lifted upward at one end to cause the other end to impact the tear panel causing it to at least partially separate from the rest of the can end. The tab and the end are separately manufactured components, the tab being attached to the end by an integral rivet formed out of the parent material of the can end.
Easy-open can ends are manufactured in reciprocating presses known as conversion presses. A conversion press converts thin metal discs called "shells" into can ends, including the integral rivet and tear panel, and concurrently forms operating tabs from a separate strip of thin metal and attaches the tabs to the ends via the integral rivets. The finished product is an easy-open can end, which may later be attached to a can body following a canning operation.
A conversion press commonly has a lower stationary press member which supports a series of lower "end" tools for performing work operations on the shells, and an upper reciprocating press member which supports a like number of upper "end" tools for performing work operations on the shells. Each pair of upper and lower end tools defines a press "end station", and a typical conversion press has five or more such stations for converting shells into can ends. Shells are carried through the various end stations by a shell conveyor which alternately moves the shells forward and then brings them to rest into alignment with the end stations so that work operations can be performed on the shells. In synchronization with the intermittent motion of the shell conveyor, the reciprocating press member alternately moves toward the stationary press member to sandwich the shells between the upper and lower end tools and thereby perform work operations upon them, and then moves away from the stationary press member to disengage the end tools from the shells so that the shells may each be advanced to succeeding work stations.
A conversion press further includes a "downstacker" apparatus which supplies shells to the shell conveyor. The downstacker typically supports one or more stacks of shells and sequentially dispenses the shells into the intermittently moving shell conveyor in synchronization with the conveyor motion.
With each "stroke" of the reciprocating press member, a given shell is subjected to a single work operation. Conversion of the shell into a can end having an integral rivet and tear panel typically requires at least five separate work operations. The shell is "indexed" through the five or more press stations, moving from one station to the next station during each upward stroke of the reciprocating press member. One of the press stations is dedicated to attaching or "staking" an operating tab to the can end via an integral rivet formed in the can end.
The tabs are made within the conversion press via a progression of upper and lower tab-forming tooling supported on the reciprocating and stationary press members, respectively. The tab-forming tooling is likewise arranged into a series of stations, along a path separate from the end tooling path. A strip of thin metal tab stock is fed through the series of tab stations, emerging from the last station as a strip of fully formed tabs interconnected by a skeleton of stock material. This strip of tabs is fed to tab-staking tooling at the tab-staking station, where each tab is separated from the strip and attached to a can end.
Because easy-open can ends are in high demand, press designers are constantly striving to increase the speed of conversion presses. This has led to the development of belt-based conversion presses in which shells are carried through the press by an apertured endless belt. Belt-based presses are capable of operating at higher speeds than rotary-type presses which carry the shells through the work stations along a circular-arc path by means of a "starwheel". The desire to increase production rate has further led to the development of "multiple-out" belt-based presses which operate upon two or more lanes of shells at the same time, and are therefore capable of producing two or more times as many can ends per minute as a "one-out" press operating at the same stroke rate.
Manufacturers of easy-open can ends typically produce ends of various sizes for various types of food and beverage cans. For example, currently, can ends ranging from about 1-inch diameter to about 4-inch diameter are commonly used in the canning industry. Thus, high-volume producers of can ends often employ a number of multiple-out belt-based presses, with each press being typically configured to manufacture can ends of only one size. However, these multiple-out presses are quite expensive, and consequently, low-volume producers of can ends often find that it is economically not practical to purchase several such presses in order to manufacture can ends of various sizes.
One way to solve this problem is to employ one press and reconfigure it to run different can end sizes. Although this solution is workable, substantial changes must be made to a typical single-size dedicated press in order to reconfigure it to process shells of a different size. For instance, a press is configured to have a specific "index" (i.e., center-to-center spacing between adjacent end stations) corresponding to the shells to be processed such that the shells are sufficiently spaced apart along the belt to have sufficient belt material between adjacent shell-carrying apertures and to have sufficient end tool working room, but close enough together to attain maximum throughput. If a different size shell is to be converted in the press, the index may have to be changed. Thus, when reconfiguring a press that is designed to process 2-inch diameter shells, which typically has a press index of about 3 inches, to enable the press to process 4-inch diameter shells, the press index necessarily must be increased to a value greater than 4 inches. Changing the press index, however, necessitates substantial changes in hardware, including the end tooling, the dies plates for mounting the end tooling on the press members, the endless belt, and the intermittent drive unit which drives the belt. Furthermore, changing end diameter frequently necessitates relocating the tooling for making the operating tabs as well as the tooling for staking the tabs onto the ends, since the integral rivet is usually not in the center of the end and therefore a change in end diameter translates into a change in rivet location. Such a change in tab tooling typically would require completely redesigning the mounting for the tab tooling on the press members.
Furthermore, different can end sizes often require different operating tabs. In order to minimize scrap during manufacturing of the operating tabs within a multiple-out conversion press, the interconnected tabs of the tab strip are advantageously located as close together as possible both in the direction along the strip and in the direction perpendicular thereto. Typically the most economical arrangement results in the tabs in adjacent rows being slightly staggered with respect to each other. As is well known, the resulting configuration of the tab strip completely determines the required "spread" of adjacent end tooling lanes (i.e., the center-to-center distance between adjacent lanes in the direction perpendicular to the direction of belt travel) as well as the "offset" of the lanes (i.e., the distance in the direction of belt travel between the center of an end station in one lane and the center of the corresponding end station in the adjacent lane). Reconfiguring a multiple-out press to run a different shell size with a different operating tab therefore frequently results in a change in both the "spread" and "offset" of the lanes. These changes in turn necessitate a complete redesign of the end tooling mounts.
Moreover, reconfiguring a typical press necessitates significant changes to the downstacker. A typical downstacker in a belt-based press includes a number of guide chutes corresponding to the number of end tooling lanes. Associated with each guide chute are typically two helical feed screws which engage the edge of the lowermost shell in a stack of shells and rotate to advance the shell downward through a feed opening at the lower end of the guide chute overlying a belt aperture. Reconfiguring a typical conversion press downstacker necessitates substantial changes in the downstacker structure for mounting the feed screws and their associated shafts and drive pulleys.
Apart from the problems associated with reconfiguring prior conversion presses, another significant problem with such presses is the balancing of press loads. The loads exerted on shells in a conversion press may reach several tons at certain end stations, particularly those stations in which the integral rivet is formed, while at other end stations the loads may be a fraction of a ton. In order to reduce wear and tear on the drive system components which drive the reciprocating press member as well as to maintain safe operation of the press, it is desirable to minimize the moments experienced by the reciprocating member about its center. To minimize moments, it is desirable to arrange the end stations and tab stations in such a manner that the total moments resulting from the sums of the moments produced at each of the individual end and tab work stations are minimized. In many conventional conversion presses, however, the path along which the tab strip travels to the tab-staking location is oriented normal to the path traveled by the can ends. The path defined by the tab-forming tooling typically is also oriented normal to the end path. The tab strip must travel between the two press supports on one side of the press during both the tab-forming and the tab-staking phases, since all tab-forming and tab-staking operations are preferably performed within the "footprint" of the press defined by the press supports. As a consequence, typically the tab-staking location in many such prior conversion presses must necessarily be approximately centrally located with respect to the press supports. Moreover, since the end must have the tear panel and the button which will become the integral rivet fully formed before the tab-staking operation can be performed, the high-load rivet-forming operations in such presses must of necessity be located toward the upstream press support. This results in difficulty balancing the loads in such presses.